In the course of drilling, completing, or servicing a subterranean well for hydrocarbon production, one or more seals, or packers, may be installed in the wellbore to isolate one zone from another. A seal may be run into a wellbore via wireline, slick line, coiled tubing, drill string, or another conveyance, and then radially expanded into sealing engagement with the interior surface of a casing, liner, or other tubular member.
Expandable seals must be able to operate against increasingly higher pressures and axial forces. Differential pressures across a seal may reach up to 15,000 psi.
Resilient materials such as rubber, which can readily be axially compressed to cause their diameters to expand, tend to have very low pressure holding capabilities due to the tendency of the resilient material to axially extrude into an extrusion gap under a differential pressure. These types of sealing mechanisms usually require structural extrusion limiters to reduce the extrusion gap. Moreover, resusable resilient seals may become subject to damaged sealing surfaces, referred to as nibbling, in which small edge portions of the sealing element become detached over repeated uses.
Recently, dissolving frac plugs have been commercialized. Currently dissolving elastomeric elements used on dissolving frac plugs tend to dissolve too slowly at temperatures below 200 F. Metallic dissolving materials dissolve more quickly below 200 F. In dissolving frac plug applications, the use of metallic seal made from the dissolving metal alloys may improve full dissolution of the frac plug below 200 F. The dissolving metallic alloys also dissolve into solution and do not reform at cooler temperatures. Currently dissolving rubber or rubber-like elements do not completely dissolve into solution; rather they flake apart into particles and chunks. Certain types may also break down to consistency of low torque grease or syrup, and in some cases these types of materials can reform as solids at the cooler temperatures that occur near the surface of the wellbore. Particles, chunks, low torque grease, syrup, or reformed solid chunks flowing through wellhead or surface equipment may create restrictions and clogs. The dissolving metallic alloys reduce this risk, because they dissolve more fully into solution.
A metallic circular seal may also be radially expanded to form a seal, which may be operable under a higher differential pressure than a resilient member and less prone to nibbling effects. However, the expansion process to a larger diameter introduces internal stresses in the sealing element. The outer diameter fibers of the seal will require a growth in length, which creates geometry and stress challenges when using metallic materials. Such stresses may cause a metallic seal to become plastically deformed and therefore not able to retract when desired. Accordingly, when designing a seal, materials and/or geometries that allow high expansion with acceptable stresses tend to sacrifice the strength and pressure holding capabilities.
For many oil and gas applications, a perfect seal is not required. A metallic circular seal that is radially expanded may prevent majority of flow and meet the application needs. In many applications, the fluid media itself may bridge seal imperfections, and in doing so, provide a full seal. For example, many hydraulic fracturing applications include sand in the fluid media. Sand and gel-like fluids used in the hydraulic fracturing media are known to bridge and block seal imperfections.